Manholes in aircrafts provide access to the fuel tank. Manholes comprise an inner fuel tank access cover (inner FTAC), an outer FTAC and a void area between the two covers. Fuel Tank Access Covers (FTACs) are mechanically fastened and clamped against the aircraft wing skin to provide fuel tank access sealing. FTAC are designed to meet a wide array of requirements, some of them are: no fuel leaks, fire resistance, resistance to a tire impact, resistance to impacts resulting from an UERF (Uncontained Engine Rotor Failure), EMH/lightning strike, seal friction, and wing bending. Despite the “no fuel leaks” requirement, in the worst case scenario from an operational and certification standpoint, it is not uncommon that seals may leak if the FTAC is installed incorrectly. Therefore, it is impossible to guarantee 100% that small amounts of fuel will never under any circumstance be present in the void zone between covers. This may also happen when FTAC are dismantled on ground and fuel drips and saturates the area until the FTAC is reinstalled. If a lighting strike or an electric static discharge occurs, an explosion can take place inside the void area of the FTAC. The current design approach to address this is to contain the explosion of the fuel/air mixture within the void zone between the inner and outer FTAC. The problem with this approach is that the FTACs require a structure with high stiffness to prevent it from being deformed after an explosion in the void area. Said deformation of the inner FTAC or the outer FTAC or both would compromise the seal integrity between the inner FTAC and the lower wing skin. In essence, currently pressure vessels are created with no relief valves.
The mentioned prior art approach is seen in the patent U.S. Pat. No. 4,291,816 wherein a fluid tight closure for an aperture, adapted to form a fuel tank access door for an aircraft, and providing fail-safe features and resistance to lightning strikes is described.